class Solution {
    int dx[4] = { 0, 0, 1, -1 };
    int dy[4] = { 1, -1, 0, 0 };
public:
    int maxDistance(vector<vector<int>>& grid) {
        int m = grid.size(), n = grid[0].size();
        vector<vector<int>> dist(m, vector<int>(n, -1));
        queue<pair<int, int>> q;

        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                if (grid[i][j] == 1)
                {
                    dist[i][j] = 0;
                    q.push({ i, j });
                }

        while (q.size())
        {
            auto [a, b] = q.front();
            q.pop();
            for (int i = 0; i < 4; i++)
            {
                int x = a + dx[i], y = b + dy[i];
                if (x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {
                    dist[x][y] = dist[a][b] + 1;
                    q.push({ x, y });
                }
            }
        }

        int ret = 0;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                ret = max(ret, dist[i][j]);

        return ret == 0 ? -1 : ret;
    }
};